Optical method and arrangement for measuring a periodic value having at least one frequency component

ABSTRACT

Polarized measuring light propagates through a sensor device and is then split into two differently linearly polarized partial light signals. An intensity-normalized measuring signal is derived from the two partial light signals and their direct components.

FIELD OF THE INVENTION

The present invention relates to a method and an arrangement formeasuring a periodic quantity. A periodic quantity is used herein todescribe a measurable quantity which, in its frequency spectrum, onlyhas frequency components that differ from zero and is, thus, inparticular, a measurable quantity that varies with time.

BACKGROUND OF THE INVENTION

PCT Application No. W/O 95/10046, describes optical measuringarrangements and measuring methods for measuring a periodic quantity, inparticular for measuring a magnetic alternating field or an electrica.c. current, utilizing the magnetooptic Faraday effect, or formeasuring an electric alternating field or an electric a.c. voltageutilizing the electrooptical Pockels effect. Polarized measuring lightis coupled into a sensor device that is under the influence of theperiodic quantity. The polarization of the measuring light is varied inthe sensor device as a function of the periodic quantity. To analyzethis change in polarization, after propagating at least once through thesensor device, the measuring light is split into two linearly polarizedpartial light signals having different polarization planes. Anintensity-normalized signal P is formed, which corresponds to thequotient of a difference and the sum of the light intensities of the twopartial light signals. A temperature-compensated measuring signal isderived from an alternating signal component and from a direct signalcomponent of the intensity-normalized signal. In this context, thedirect signal component does not contain any frequency components of theperiodic quantity and is only used for temperature compensation.

“Optical Combined Current & Voltage H.V. Sensors, GEC Alsthom, T&D,describes a magnetooptical current transformer in which a light signalthat is linearly polarized in a polarizer propagates through a Faradayglass ring and is then split by a polarizing beam splitter into twopartial light signals, which are linearly polarized, transversely withrespect to one another (two-channel polarization analysis). Each of thetwo partial light signals is fed via an optical fiber to a correspondingphotodiode, which converts the partial light signal in question into anelectric intensity signal S1 or S2, which is proportional to the lightintensity of the corresponding partial light signal. Due to thedifferent attenuation in the two optical fibers, the two proportionalityconstants can differ from one another at this point. To compensate forthese differences in responsivity, provision is made for a specialclosed-loop control. A controllable first amplifier connected downstreamfrom the first photodiode amplifies the intensity signal S1 by acorresponding gain K1, and a second amplifier connected downstream fromthe second photodiode amplifies the second intensity signal S2 by asecond gain K2. At this point, direct signal components (DC values) ofthe two intensity signals S1 and S2 are determined, and the differencebetween the two direct signal components is set to zero by controllingthe gain K1 of the first amplifier. From the two intensity signals K1·S1and K2·S2, which are generally amplified with varying intensity, at theoutputs of the two amplifiers, a measuring signal is now formed, whichcorresponds to the quotient (K1·S1−K2·S2)/(K1·S1+K2·S2) of thedifference and the sum of the output signals of the amplifiers.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical measuringmethod and an optical measuring arrangement for measuring a periodicquantity, where the polarization state of polarized measuring light in asensor device is varied as a function of the periodic quantity, and themeasuring light for analyzing this change in polarization is split,after propagating through at least once, into two variably linearlypolarized, partial light signals, and undesired intensity variations inthe light paths of the measuring light and of the two partial lightsignals are compensated.

A method for measuring a periodic quantity according to the presentinvention includes the following method steps:

a) polarized measuring light propagates at least once through a sensordevice that is under the influence of the periodic quantity, the sensordevice varying the polarization of the measuring light as a function ofthe periodic quantity, and is then split into two linearly polarizedpartial light signals having light intensities I1 and I2 and differentpolarization planes;

b) from the light intensities I1 and I2 of the two partial light signalsand direct components I1 _(DC) or I2 _(DC) of these two lightintensities I1 and I2, a measuring signal is formed for the periodicquantity, which is essentially proportional to the quotient

(I2 _(DC)·I1−I1 _(DC)·I2)/(I2 _(DC)·I1+I1 _(DC)·I2),

the two direct components I1 _(DC) or I2 _(DC) not containing anyfrequency components of the periodic quantity.

An arrangement for measuring a periodic quantity according to thepresent invention includes:

a) a sensor device, which varies the polarization of polarized light asa function of the periodic quantity;

b) means for coupling polarized measuring light into the sensor device;

c) means for splitting the measuring light, after propagating at leastonce through the sensor device, into two linearly polarized partiallight signals having different polarization planes and having lightintensities I1 or I2;

d) means for generating a measuring signal for the periodic quantityfrom light intensities I1 and I2 of the two partial light signals anddirect components I1 _(DC) or I2 _(DC) of these two light intensities I1or I2, which do not contain any frequency components of the periodicquantity, the measuring signal essentially being proportional to thequotient

(I2 _(DC)·I1−I1 _(DC)·I2)/(I2 _(DC)·I1+I1 _(DC)·I2).

Because of the special consideration given to the direct signalcomponents I1 _(DC) and I2 _(DC) of the two light intensities I1 or I2as an index for the mentioned intensity variations in the light paths,the measuring signal is virtually completely intensity-normalized.

Accordingly, the method and the arrangement are preferably used in afirst advantageous specific embodiment for measuring a magneticalternating field, in that a sensor device indicating the magnetoopticalFaraday effect is used, and the measuring signal is retrieved as anindex for the magnetic alternating field.

In a second advantageous specific embodiment, the method and arrangementfor measuring an electric a.c. voltage or an electric alternating fieldin which a sensor device indicating the electrooptical Pockels effect isused, and the measuring signal is retrieved as an index for the electrica.c. voltage or for the electric alternating field.

The two partial light signals are preferably transmitted in each casevia at least one optical fiber and, in particular, via at least twooptical fibers and one optical connector for detachably joining the twooptical fibers. The connectors are advantageously used for temporarilydisconnecting the sensor device that is generally linked to differentelectric potentials, on the one hand, and the evaluation electronics, onthe other hand. In this specific embodiment, the measuring signal isalso independent of light intensity variations in the two partial lightsignals in response to variations in the attenuation properties of theconnectors following their opening and subsequent closing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of a measuring arrangement formeasuring a magnetic alternating field, in particular of the magneticalternating field of an electric a.c. current; and

FIG. 2 shows an exemplary embodiment of a measuring arrangement formeasuring an electric a.c. voltage.

DETAILED DESCRIPTION

FIG. 1 depicts an optical measuring arrangement for measuring a magneticalternating field H, in particular for measuring an electric a.c.current I in a current conductor 2. A Faraday sensor device 3 isallocated to magnetic alternating field H. Sensor device 3 is made of anoptical waveguide, preferably an optical fiber, which surrounds currentconductor 2 in a measuring winding having at least one measuring turn,and which exhibits the magnetooptical Faraday effect. As Faraday sensordevice 3, provision can also be made, however, for one or a plurality ofsolid bodies made of a Faraday material, which form(s) a light path,preferably a glass ring, preferably surrounding current conductor 2. Itis also not necessary for Faraday sensor device 3 to surround currentconductor 2 in a closed light path; it may also be merely disposed inthe proximity of current conductor 2, within magnetic field H of a.c.current I.

Linearly polarized measuring light L is preferably coupled via apolarization-maintaining optical waveguide 34 into sensor device 3. Toproduce this linearly polarized measuring light L, provision can be madefor a light source and allocated polarizing means (not shown), or alsofor a self-polarizing light source 4, for example a laser diode and, ifindicated, additional polarizing means (not shown). The linearlypolarized measuring light L propagates at least once through sensordevice 3 and is subjected, in the process, to a Faraday rotation ρ ofits polarization plane as a function of magnetic alternating field H orof electric a.c. current I. After propagating through sensor device 3,measuring light L is fed to an analyzer 7 and is split in analyzer 7into two linearly polarized partial light signals L1 and L2, whosepolarization planes differ from one another. The polarization planes ofthe two partial light signals L1 and L2 are preferably alignedtransversely to one another (orthogonal splitting). As analyzer 7,provision can be made for a polarizing beam splitter, for example aWollaston prism, or also for a simple beam splitter having asemi-reflective mirror with two polarization filters, which areoptically coupled downstream and crossed at an appropriate angle,preferably 90°. Sensor device 3 and analyzer 7 can be optically coupledto one another via a free-beam arrangement or also via apolarization-maintaining optical waveguide 37, preferably a monomodeoptical waveguide, such as a HiBi (high birefringence) fiber or apolarization-neutral LoBi (low birefringence) fiber. The opticalwaveguide of sensor device 3 is connected to optical waveguide 34 forsupplying measuring light L, and to optical waveguide 37 for removingmeasuring light L, in each case preferably via a splice 38 or 39.

In one specific embodiment (not shown), measuring light L, afterpropagating through a first time, is reflected back into Faraday sensordevice 3, and propagates through Faraday sensor device 3 a second timein the reverse direction (reflection type), before being split intopartial light signals L1 and L2.

The two partial light signals L1 and L2 are each fed to a photoelectrictransducer 12 or 22, preferably in each case to a photodiode arranged inan amplifier circuit. As described, the two partial light signals L1 andL2 can be transmitted from analyzer 7 to transducer 12 or 22 in questionvia a free-beam arrangement or, in each case, via an optical waveguide.The first photoelectric transducer 12 converts first light signal L1into a first electric intensity signal S1, which is essentiallyproportional to light intensity I1 of first light signal L1, thusS1=K1·I1. Second photoelectric transducer 22 converts second lightsignal L2 into a second electric intensity signal S2, which isessentially proportional to light intensity I2 of second light signalL2, thus S2=K2·I2. Proportionality factors K1 and K2 of theseconversions are determined by the photoelectric efficiency and by thesubsequent amplifications of the signals in transducers 21 and 22, andcan also change over time due to interference effects.

The two intensity signals S1 and S2 are analyzed at this point in anevaluation unit 20, preferably in the following manner. Each of the twoelectric intensity signals S1 and S2 is fed to an input of acorresponding first multiplier 23 or to a second multiplier 24, and toan input of a corresponding first filter 28 or second filter 29 havinglow-pass character. First filter 28 generates a direct signal componentD1 of first intensity signal S1, which corresponds to the K1-fold directcomponent I1 _(DC) of light intensity I1 of first light signal L1, thusD1=K1·I1 _(DC). Direct signal component D1 of first intensity signal S1active at an output of filter 28 is fed to a second input of firstmultiplier 23. Second filter 29 generates a direct signal component D2of second intensity signal S2, which corresponds to the K2-fold directcomponent I1 _(DC) of light intensity I2 of second light signal L2, thusD2=K2·I2 _(DC). Direct signal component D2 of second intensity signal S2active at an output of second filter 29 is fed to a second input ofsecond multiplier 24. As filters 28 and 29, analog or digital low-passfilters can be used, for example, whose separation frequencies areadjusted to be lower, in each case, than the lowest frequency in thespectrum of the periodic quantity, thus, in the depicted specificembodiment, of magnetic alternating field H or of electric alternatingcurrent I. The two direct signal components D1 and D2 and, thus, alsothe two direct light components I1 _(DC) and I2 _(DC) do not contain anyinformation about the periodic quantity (in particular the magneticalternating field H), however they do contain the information about anundesirable operating point drift of the two light intensities I1 andI2. This information about an intensity drift is now used, as follows,for deriving an intensity-normalized measuring signal. First multiplier23 forms the product D2·S1 of first intensity signal S1 and of directsignal component D2 of second intensity signal S2. Second multiplier 24forms the product D1·S2 of direct signal component D1 of first intensitysignal S1 and of second intensity signal S2. These two products D2·S1and D1·S2 are now fed from the output of corresponding multiplier 23 or24, in each case to an input of a subtracter 25 and an input of an adder26. Differential signal D2·S1−D1·S2 of the two product signals D2·S1 andD1·S2 formed by subtracter 25 is applied to a first input of a divider27. The composite signal D2·S1+D1·S2 of the two products D2·S1 and D1·S2formed by adder 26 is active at the second input of divider 27. At anoutput of divider 27, the measuring signal

M=(D2·S1−D1·S2)/(D2·S1+D1·S2)  (1)

can now be tapped off for the magnetic alternating field H or for theelectric alternating current I corresponding to the quotient signal fromthe difference D2·S1−D1·S2 and the sum D2·I1+D1·I2.

In a slightly altered specific embodiment (not shown) of the signalevaluation in evaluation unit 20, a quotient of two direct signalcomponents D1 and D2 is initially determined as correction factorK=D1/D2. This correction factor K is used to form a measuring signal

M′=(S1−K·S2)/(S1+K·S2)  (2).

The two measuring signals M according to equation (1) and M′ accordingto equation (2) are both equivalent to the quotient formed directly fromlight intensities I1 and I2 and their direct components I1 _(DC) and I2_(DC):

M=M′=(I1·I2 _(DC)−I2·I1 _(DC))/(I1·I2 _(DC)+I2·I1 _(DC))  (3)

Thus, the responsivities K1 and K2 of the two transducers 12 and 22 dropout when measuring signal M according to equation (1) or measuringsignal M′ according to equation (2) is formed.

One advantage of the described specific embodiment of evaluation unit 20having analog arithmetic modules is rapid signal processing. Of course,measuring signal M or M′ can also be ascertained with the aid of a tableof values and/or with the aid of digital modules.

In addition, measuring signal M or M′ is virtually completelyintensity-normalized according to one of equations (1) to (3). Thismeans that undesired changes in light intensities I1 and I2 of the twopartial light signals L1 and L2 caused by transmission losses no longerhave an effect on measuring signal M or M′.

At this point, from measuring signal M or M′, electric a.c. current I incurrent conductor 2 can be determined with the aid of the equation ρ=N VI, V being the Verdet constant of the Faraday effect in sensor device 3,and N the number of revolutions of measuring light L around currentconductor 2.

FIG. 2 depicts a specific embodiment of an optical measuring arrangementfor measuring an electric a.c. voltage U as periodic quantity Xincluding a sensor device 3 for indicating the electrooptical Pockelseffect. A.c. voltage U to be measured is able to be applied via twoelectrodes 35 and 36 to Pockels-sensor device 3′. Polarized measuringlight L is coupled into Pockels-sensor device 3′. This measuring light Lpropagates through Pockels-sensor device 3′ and is subjected, in theprocess, to a change in its polarization as a function of applied a.c.voltage U. In the depicted specific embodiment, a.c. voltage U isapplied transversely to the light propagation direction of measuringlight L (transversal specific embodiment), but can also be applied inparallel to the light propagation direction (longitudinal specificembodiment). As means for coupling measuring light L into sensor device3′, provision is made for a light source 4, for example a light-emittingdiode, and a polarizer 5 for linearly polarizing the light from lightsource 4. Light source 4 and polarizer 5 are preferably opticallycoupled to one another via an optical waveguide 43, for example amultimode optical fiber, but can also be optically coupled to oneanother by a free-beam coupling. To couple the light from opticalwaveguide 43 into polarizer 4, provision is preferably made for acollimator lens (e.g., grade-index lens [i.e., GRIN lens]) 25. Measuringlight L, which is linearly polarized at this point, is coupled frompolarizer 5 into Pockels-sensor device 3′. After propagating throughPockels-sensor device 3, measuring light L is fed via a λ/4 platelet(quarter-wave platelet) 6 to analyzer 7. In analyzer 7, measuring lightL is split into two linearly polarized partial light signals L1 and L2,whose planes of polarization differ from one another. The planes ofpolarization of the two partial light signals L1 and L2 are preferablydirected transversely to one another (orthogonal splitting). As analyzer7, provision can be made for a polarizing beam splitter, for example aWollaston prism, or also for two polarization filters, which are crossedby a predefined angle, preferably 90°, and for an upstream, simple beamsplitter.

The operating point of the measuring arrangement according to FIG. 2 ispreferably adjusted in such a way that circularly polarized measuringlight is applied to analyzer 7 when no electric field is applied toPockels-sensor device 3′. The two intrinsic axes of the linearbirefringence in Pockels-sensor device 3′ are “uniformly illuminated” inthis case by measuring light L. This means that the components ofmeasuring light L projected onto the two intrinsic axes exhibit the sameintensity. Generally, then, the two partial light signals L1 and L2 arelikewise of equal intensity. When an a.c. voltage (U≠0V) is applied toPockels-sensor device 3′, the intensity of the components of measuringlight L along the electrooptically active intrinsic axes of the linearbirefringence of Pockels-sensor device 3′ is altered as a function ofa.c. voltage U.

In place of the optical series connection, as shown in FIG. 2, ofpolarizer 5, of Pockels-sensor device 3′, of λ/4 platelet 6, and ofanalyzer 7, provision can also be made for an optical series connectionof polarizer 5, of λ/4 platelet 6, of Pockels-sensor device 3′, and ofanalyzer 7, thus the order of λ/4 platelet 6 and of sensor device 3′would be exactly reversed. In this case, measuring light L is circularlypolarized before being coupled into Pockels-sensor device 3′. Moreover,in place of light source 4 and polarizer 5, provision can also be madefor a light source for transmitting linearly polarized light, such as alaser diode, for coupling polarized measuring light L into sensor device3′ or λ/4 platelet 6. Optical waveguide 43 is then preferably apolarization-maintaining optical waveguide. In addition, partial lightsignals L1 or L2 can also be transmitted in a free-beam arrangement.Moreover, analyzer 7 can be optically coupled to λ/4 platelet 6 or toPockels-sensor device 3′ via a polarization-maintaining opticalwaveguide.

The two partial light signals L1 and L2 are preferably coupled via acollimator lens 11 and 21, respectively, into an optical waveguide 13and 16, respectively. Optical waveguides 13 and 16 are each connectedvia an optical connector 14 and 17, respectively, to another opticalwaveguide 15 and 18, respectively. Sensor device 3 is detachable fromevaluation unit 20 by way of connectors 14 and 17. At this point, thetwo partial light signals L1 and L2 are coupled via correspondingconnectors 14 and 17 and the corresponding, other optical waveguides 15or 18. After propagating through Pockels-sensor device 3, measuringlight L is fed via a λ/4 platelet 6 to analyzer 7. In analyzer 7,measuring light L is split into two linearly polarized partial lightsignals L1 and L2, whose planes of polarization differ from one another.The planes of polarization of the two partial light signals L1 and L2are preferably directed transversely to one another (orthogonalsplitting). As analyzer 7, provision can be made for a polarizing beamsplitter, for example a Wollaston prism, or also for two polarizationfilters, which are crossed by a predefined angle, preferably 90°, andfor a simple beam splitter. Provision can also be made for theconnectors in all other specific embodiments of the measuringarrangement, in particular in the one depicted in FIG. 1.

The two electric intensity signals S1 and S2 are digitized with the aidof analog/digital converter 30, and the digitized signals are processedfurther by a microprocessor or a digital signal processor 40 to generatea measuring signal M according to equation (1) or M′ according toequation (2). Analog/digital converter 30 and processor 40 then formevaluation unit 20. Processor 40 filters direct signal components D1 andD2 in a digital operation, and then calculates measuring signal M or M′according to equation (1) or (2).

What is claimed is:
 1. A method for measuring a periodic variable havingat least one frequency component, the method comprising the steps of:propagating a polarized measuring light having a polarization at leastonce through a sensor device, the sensor device being under an influenceof the periodic variable; varying by the sensor device the polarizationof the polarized measuring light as a function of the periodic variable;splitting the polarized measuring light into a first linearly polarizedpartial light signal and a second linearly polarized partial lightsignal, the first linearly polarized partial light signal having a firstpolarization plane and a first light intensity, the first lightintensity having a first direct component, the second linearly polarizedpartial light signal having a second polarization and a second lightintensity, the second light intensity having a second direct component;generating a first proportional electric intensity signal as a functionof the first light intensity of the first linearly polarized partiallight signal; generating a second proportional electric intensity signalas a function of the first direct component of the first lightintensity; generating a third proportional electric intensity signal asa function of the second light intensity of the second linearlypolarized partial light signal; generating a fourth proportionalelectric intensity signal as a function of the second direct componentof the second light intensity; and generating a measuring signal as afunction of the first proportional electric intensity signal, the secondproportional electric intensity signal, the third proportional electricintensity signal, and the fourth proportional electric intensity signal,the measuring signal being proportional to the following quotient: (I2_(DC)·I1−I1 _(DC)·I2)/(I2 _(DC)·I1+I1 _(DC)·I2), wherein I1 is the firstlight intensity of the first linearly polarized partial light signal,wherein I1 _(DC) is the first direct component of the first lightintensity, wherein I2 is the second light intensity of the secondlinearly polarized partial light signal, wherein I2 _(DC) is the seconddirect component of the second light intensity, wherein the firstpolarization plane of the first linearly polarized partial light signalis different from the second polarization plane of the second linearlypolarized partial light signal, and wherein the first direct componentand the second direct component do not contain any one of the at leastone frequency components of the periodic variable.
 2. The methodaccording to claim 1, wherein the periodic variable includes a magneticalternating field, the sensor device includes a Faraday effect sensingdevice, and the measuring signal indicates an index for the magneticalternating field.
 3. The method according to claim 1, wherein theperiodic variable includes at least one of an electric a.c. voltage andan electric alternating field, the sensor device includes a Pockelseffect sensing device, and the measuring signal indicates an index forthe at least one of the electric a.c. voltage and the electricalternating field.
 4. An arrangement for measuring a periodic variablehaving at least one frequency component, comprising: a sensor devicereceiving through a coupling member a polarized measuring light having apolarization, the polarized measuring light propagating at least oncethrough the sensor device, the sensor device varying the polarization ofthe polarized measuring light as a function of the periodic variable asplitter device for splitting the polarized measuring light into a firstlinearly polarized partial light signal and a second linearly polarizedpartial light signal, the first linearly polarized partial light havinga first polarization and a first light intensity, the first lightintensity having a first direct component, the second linearly polarizedpartial light having a second polarization and a second light intensity,the second light intensity having a second direct component; a firstcircuit arrangement generating: a first proportional electric intensitysignal as a function of the first light intensity of the first linearlypolarized partial light signal, a second proportional electric intensitysignal as a function of the first direct component of the first lightintensity, a third proportional electric intensity signal as a functionof the second light intensity of the second linearly polarized partiallight signal, and a fourth proportional electric intensity signal as afunction of the second direct component of the second light intensity;and a second circuit arrangement generating a measuring signal as afunction of the first proportional electric intensity signal, the secondproportional electric intensity signal, the third proportional electricintensity signal, and the fourth proportional electric intensity signal,the measuring signal being proportional to the following quotient: (I2_(DC)·I1−I1 _(DC)·I2)/(I2 _(DC)·I1+I1 _(DC)I2), wherein I1 is the firstlight intensity of the first linearly polarized partial light signal,wherein I1 _(DC) is the first direct component of the first lightintensity, wherein I2 is the second light intensity of the secondlinearly polarized partial light signal, and wherein I2 _(DC) is thesecond direct component of the second light intensity, wherein the firstpolarization of the first linearly polarized partial light is differentfrom the second polarization of the second linearly polarized partiallight, and wherein the first direct component and the second directcomponent do not contain any one of the at least one frequencycomponents of the periodic variable.
 5. The arrangement according toclaim 4, wherein the periodic variable includes a magnetic alternatingfield, and the sensor device includes a magnetooptical Faraday effectsensing device.
 6. The arrangement according to claim 4, wherein theperiodic variable includes at least one of an electric a.c. voltage andan electric alternating field, and wherein the sensor device includes anelectrooptical Pockels effect sensing device.
 7. The arrangementaccording to claim 4, further comprising at least one optical fiberproviding the first and the second linearly polarized partial lightsignal to the first circuit arrangement.
 8. The arrangement according toclaim 7, wherein the at least one optical fiber includes at least twooptical fibers, and the arrangement further comprising an opticalconnector coupling each one of the at least two optical fibers toanother one of the at least two optical fibers.